This invention relates to a laser printer.
A laser printer comprises means for radiating a laser beam, modulator means for modulating the laser beam in accordance with image signals, and deflector means comprising a rotary polyhedral mirror for deflecting the path of the laser beam modulated in accordance with the image signals to scan the surface of a photosensitive member. The laser beam deflected by the deflector means is incident on the surface of the photosensitive member to carry out scanning thereof to effect exposing of the photosensitive member to an image of a document and form an electrostatic latent image thereon. The latent image is developed into a visible image to produce a print. It has hitherto been usual practice to use a photosensitive member in the form of a drum.
To meet the requirements for increasing space efficiency and obtaining an overall compact size in a laser printer, however, proposals have recently been made to use a photosensitive member in the form of an endless belt. It has been found that when a photosensitive member of the drum type is replaced by a photosensitive member of the endless belt type, trouble occurs that has not been associated with the photosensitive member of the drum type.
More specifically, it has been found that the endless belt tends to undulate in wave form in operation, thereby causing variations in the belt travelling speed and loosening of the belt to occur locally.
When the laser beam is incident on the surface of the photosensitive member in a position midway between the photosensitive belt support rollers, the formation of an image on the photosensitive member is adversely affected by the aforesaid waving phenomenon, local variations in belt travelling speed and loosening of the belt.
When the photosensitive member moves in wave form or slackens in a position in which the charger or the developing sleeve of the developer is juxtaposed against the photosensitive member, variations occur in the corona discharge distance and developing gap and the charging performance and developing performance are adversely affected.
A rotary polyhedral mirror of the regular polygonal shape is widely used as a laser beam deflector. When a laser beam is caused to be incident on the rotary polyhedral mirror at a right angle to the axis of rotation of the rotary polyhedral mirror, the light scattered by the surface of the photosensitive member is reflected and travels backwardly to fall on the rotary polyhedral mirror again. The reflected light forms a stationary ghost image on the surface of the photosensitive member. In an effort to avoid the phenomenon of ghost image formation, an attempt has been made to cause a laser beam to be incident on the surface of the photosensitive member in the form of an endless belt in an oblique direction with respect to the ancillary scanning direction while allowing the incident rays and the axis of rotation of the rotary polyhedral mirror to form an angle of 90 degrees, to thereby minimize reflection of the scattered light by the surface of the photosensitive member that travels backwardly and falls on the rotary polyhedral mirror again. When this process is used with a laser printer including a photosensitive member of the endless belt type, the beam incident position would show variations in the ancillary scanning direction if the photosensitive belt undulates in wave form in the laser beam incident position.